1. Field of the Invention
The present invention relates to solder paste, and in particular, a solder paste for fabricating bumps.
2. Description of the Related Art
The development of IC packaging technology moves along the miniaturized and high density trend, and in particular, with the development of high density IC packaging technology, such as flip chip packaging, the shortened signal path assists the upgrading of transmission speed.
Thus, flip chip bonding technology is gradually becoming the mainstream of high density packaging.
In common flip chip bonding technology, after the re-distribution process, a bump is formed on the bonding pads (I/O connection point) of the wafer.
The wafer is cut into a single chip which is then flipped, and a bump is then used to connect the chip to the substrate.
In the above mentioned bump fabrication, generally, the surface of the bonding pad of the wafer is first formed into an under ball metallurgy layer and then a photo sensitive dry film is adhered.
Next, the dry film is patterned and an opening is formed on the dry film. After that, the opening is filled with solder paste. Next, a re-flow process of solder paste is performed so as to form a spherical bump. Finally, the dry film is removed.
In order to cause the solder paste to smoothly become a spherical shape and to remove the unnecessary oxides and impurities within the solder paste, generally the solder paste is mixed with flux. By means of the re-flow process of solder paste, the reaction between the flux and the solder paste causes the oxides and impurities of the solder paste to be brought to the surface of the bump. After that, the flux is removed.
In the conventional re-flow process of the solder paste, voids will normally remain in the bump and the yield and reliability of the bump is reduced.
In view of the above, the drawbacks of the conventional solder paste is that in the course of bump formation, voids are formed in the bump and this will affect the yield and reliability of the bump.
Accordingly, it is an object of the present invention to provide a solder paste for fabricating bumps, which can improve the yield of the bump.
Yet another object of the present invention is to provide a solder paste for bump fabrication, which can improve the reliability of the bump.
A further object of the present invention to provide a solder paste for bump fabrication which can prevent the formation of voids in the bump.
In the current re-flow process of solder paste, the flux within the solder paste will react with the oxide of the solder paste to form gases and voids are formed in the bump. This will lower the yield and reliability of the solder paste.
In order to achieve the above and other objects, the invention provides a solder paste for fabricating bumps comprising a flux and metallic alloy powder. The metallic alloy powder includes a plurality of low eutectic metallic alloy granules, and the size of these metallic alloy granules is 20-60 xcexcm and the average size of the metallic granules is 35 to 45 xcexcm.
In accordance with the solder paste for fabricating bumps, the weight percentage of the metallic alloy powder with respect to the solder paste is 85-95%.
In accordance with the solder paste for fabricating bumps, the metallic alloy powder is Sn/Pb alloy formed by grinding and the ratio of Sn/Pb alloy is 94-98%.
In accordance with the present invention, the metallic alloy granules are larger than that of the conventional (the size of the conventional metallic alloy granules is 4 to 25 xcexcm and the average size of the conventional alloy granules is 12 xcexcm), when the amount of metallic alloy used is similar, the total surface area of the metallic alloy granules is smaller than the conventional metallic alloy granules. Thus, in the re-flow process in the fabrication of bumps, the total surface area of reaction of the metallic alloy granules and the flux is significantly smaller than that of the conventional metallic alloy granules and the flux.
Consequently, the present invention reduces the product of the reaction of the flux and the solder paste in the re-flow process.
Thus, the formation of voids in the bump is reduced.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.